The present invention relates to a thrust needle-shaped roller bearing, a rolling bearing, and a cage of the thrust needle-shaped roller bearing, particularly, which are suitable under such environments as using in alternate fluorocarbon and the like.
Conventionally, a thrust needle-shaped roller bearing includes a cage and needle-shaped rollers, which are often combined with raceway rings. Needle-shaped rollers are usually of such a size that the ratio of roller length (lr) to diameter (Da) ranges from about 1.4 to 3.4 (lr/Da).
The recent concerns over global warming and other possibilities of environmental destruction have forced the manufacturers of compressors in air conditioners and the like to use alternate fluorocarbon such as HFC-134a serving as refrigerants. Compared with the conventionally used CFC-12 and other chlorofluorocarbon, the alternate fluorocarbon is inferior in self-lubricating operation. In addition, PAG (polyalkylene glycol) which is used as a working oil because of its high compatibility is characterized by the great tendency to absorb moisture.
Therefore, the use of a bearing in such an environment as the alternate fluorocarbon and PAG causes a combined operation in which, first, micro-pitting occurs due to the small amount of water entering into PAG, and secondary, the viscosity of PAG is reduced due to water absorption and the dissolution of the alternate fluorocarbon so that the lubricating conditions are deteriorated. As a result, the combined operation causes various problems provided with surface damages such as flaking, wear and the like, which lead to "seizure" of the bearing and fail to secure the required life characteristics.
In order to solve these problems, it is necessary to improve not only the corrosion resistance of the bearing material but also the lubricating conditions. A common method for assuring better corrosion resistance is of using stainless steels (e.g. SUS 440C) as the bearing material. To improve the lubricating conditions, the approach of using a modified working oil is the most effective. However, since the range of selection of working oils is limited in point of compatibility, much improvement cannot be expected in lubricity.
Under the circumstances, a method for preventing the seizure is expectable of using a large amount of lubricant in the refrigerant so as to improve its lubricity. But, this method causes another problem of deterioration in the cooling performance of the compressor. A known method that has been proposed as a technique to solve this new problem is of preventing the seizure by a surface treatment to the cage, namely by forming a plasma polymerized layer of a fluorocarbon compound on a substrate (Unexamined Japanese Patent Publication No. Hei. 2-238094).
Cages for holding the rolling elements of bearings have conventionally been formed of metals. But, metallic cages suffer the problem that they are liable to wear as the result of friction with the rolling elements, leading to shorter lives. To avoid this problem, resin cages have already been proposed in Unexamined Japanese Patent Publication Nos. Sho. 64-79419 and Hei. 4-357323. The resin cages as proposed have all parts made of resin.
In a thrust needle-shaped roller bearing, its dynamic torque can be reduced by shortening the length of each roller. However, if the rollers are made too short, several problems occur such as instability in the attitude of the rollers, their falling away from the cage, and the difficulty involved in grinding the rollers.
If stainless steels are used as bearing materials with a view of improving resistance to corrosion, the cost of bearings becomes so high that they are no longer suitable for use in compressors for vehicles. As mentioned above, the approach of improving the lubricating conditions through the use of modified working oils has not achieved sufficient effects. Further, the surface treatment (i.e., the formation of a plasma polymerized layer) proposed in Unexamined Japanese Patent Publication No. Hei. 2-238094 is not suitable for mass production without increasing the cost. In addition, uniform surface treatments can be accomplished on flat objects such as substrates, but cannot be done on objects having complicated shapes such as the cage of a bearing.
Cages all made of resin have been proposed with a view of solving the problem with metallic cages that they are liable to wear as the result of friction with rollers to thereby become short-lived. However, the resin cages are less rigid than the metal cages. Particularly, in the case where they are used in thrust needle-shaped bearings, the partitions (columns) between rollers and the outer circumferential portions that receive the end faces of the rollers are so extremely thin that creep deformation occurs due to the centrifugal force of rollers rotating at high speed.